1. Technical Field
The present disclosure relates to a ceramic cathode material of a solid oxide fuel cell and a manufacturing method thereof; in particular, to a ceramic cathode material which is applicable in a fuel cell, and the ceramic cathode material has the features of higher electrical conductivity and reduced thermal expansion coefficient operating within the temperature range from 500 to 800 degrees Celsius.
2. Description of Related Art
There are multiple fuel cells that are categorized in terms of electrolyte and operating temperature range. The fuel cells with the largest development potential are the proton exchange membrane fuel cell (PEMFC) and the solid oxide fuel cell (SOFC).
One of the main objectives of commercializing the SOFC is for lowering the operating temperature of the same. Generally, the operating temperature of the high temperature SOFC ranges from 800 to 1000 degrees Celsius, but the high operating temperature accompanies some defects, such as relatively lower open circuit voltage, more demanding requirement for the cell material, more expensive material cost, and longer waiting time for rising the temperature of the connection board. In addition, because of the high operating temperature requiring the longer waiting time for increasing or decreasing the temperature, tensile stress and compressive stress in the inner structure of the fuel cell may result, increasing the possibility of components of the cell fuel being damaged. The operating temperature of the IT (intermediate)-SOFC ranges from 500 to 800 degrees Celsius. Comparing with the high temperature SOFC, the IT-SOFC has the advantages such as extended lifetime and more connection material selections (without being limited to the use of ceramic material only). However, in the relatively lower operating temperature, the electrical conductivity is lowered and the activating polarity may apparently increase along with the reduced operating temperature. Therefore, the development of the medium/low temperature cathode material with better performances is very important.
The advantages of solid oxide fuel cell are as follows:
(1) High efficiency: The conventional electricity generation processes need to go through a series of energy conversions. Part of the energy may dissipate into the air during each energy conversion, thus the total energy conversion efficiency is relatively low. The efficiency of the conventional thermal power generation is about 30%. The fuel cell directly converts the chemical energy into electricity without coal burning process, thus the energy waste thereof is relatively low. Theoretically, the efficiency of the fuel cell is around 85 to 90% despite only about 40 to 60% in practice.
(2) No noise: Presently, the common electricity generation technique including thermal electricity generation, hydro electricity generation, or nuclear electricity generation mainly uses large turbines generating large amount of noises during the operation. On the other hand, the fuel cell does not need machine parts when performing electrochemical reactions, which is different from the conventional electricity generation. Therefore, the fuel cell could be a virtually noiseless electricity generation system.
(3) Low contamination: In coal, petroleum, nuclear energy for generating electricity, harmful substances such as SOx, NOx, and COx may be generated and nuclear waste may be difficult to be disposed of. On the other hand, the fuel cell is relatively environmentally friendly option.
(4) Variety selections of fuel: Some specific fuel cell sets can use the energy other than hydrogen gas. Due to relatively lower density, the hydrogen gas is not convenient to be stored. Thus, the liquid hydrogen energy is used as fuels, such as alcohol or liquid fossil fuels, for providing much more convenience and durability.
Presently, the problems of the cathode material of the most widely used high temperature SOFC are that the electrical conductivity, the thermal expansion coefficient, and the stability thereof remain to be desired and not suitable just yet for large scale commercial development. Therefore, the cathode material needs to have relatively better electrical conductivity, electrolyte matching, and stability when operating in the intermediate temperature range. The matching materials include the perovskite, the cubic fluorite, and the pyrochlore, and the pyrochlore so far has been gaining more popularity.
The following descriptions are the must-have conditions of the medium cathode materials:
(1) Stability: The cathode material needs to have chemical, crystal form, morphology, and size stabilities, under the temperature from the room temperature to the operating temperature. In addition, the electrolyte, the connection material, and other components also need to have good chemical stability.
(2) Electrical conductivity: The cathode needs to have high ionic conductivity and electron conductivity for lowering the ohmic polarization.
(3) Thermal expansion: The thermal expansion coefficient of the material needs to match the thermal expansion of the electrolyte, connection materials, and other components, for avoiding deformation, detachment, and cracks.
(4) Porosity: For allowing the gas to reach and react at the electrode, the cathode material needs to have at least 30% of the porosity.
(5) Catalytic capability: The material needs to have good catalytic capability toward oxygen for allowing the oxygen molecule to perform dissociation reactions.
In early days, the cathode material uses the precious metals including platinum, palladium, and silver, etc., which have good electrical conductivity. However, the precious metals are expensive and the silver is volatile under high temperature. The Ln1−xAxMO3+δ (Ln is the lanthanum series element, A is alkaline earth family element, and M is the transition metal element) which has the structure of perovskite could satisfy the requirements of electrical conductivity of the cathode material. Generally, the material is made by adding alkaline earth family element into LnMO3, for increasing the electrical conductivity of the cathode material under the high temperature. By replacing part of the rare earth family element with alkaline earth family, the valence number of the transition metals changes or forms oxygen vacancies under certain conditions, for maintaining the lattice electrical neutrality and for increasing electrical conductivity.
The LaCoO3−δ is a standard perovskite structure material which is rhombohedral phase structure and forms a distorted octahedral structure (CoO69−) under the room temperature. In addition, the rhombohedral phase structure may phase transform into cubic structure at the temperature of 509 degrees Celsius. The LaCoO3−δ cathode material is the mixed conductor which has the features of electron conductivity and ionic conductivity, and is a semi-conductive material. However, although the LaCoO3−δ and LaCo0.4Ni0.6O3−δ which are manufactured by solid state-gel method have high electrical conductivity and have the potential of being applied to the cathode of the medium temperature SOFC, the thermal expansion coefficient and the sintering temperature are the areas to be improved to meet the needs.
Therefore, a possible solution for developing the cathode material of the medium/low temperature high performance SOFC may be using doping element with similar atomic radius for replacing Ni and Co in order to increase the generation of the oxygen vacancies and to reduce the thermal expansion coefficient, by using solid synthesis to manufacture, and by executing microstructure and electrical analysis after selecting the optimal parameters for developing new cathode material with better features and lower cost.